World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 5
T E C H N I C A L
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M A N U A L
minimised, the combination of them, and the very small nature of the ionisation
current (10 - 50 picoamps), limit the sensitivity and usefulness of ionisation detectors.
It should also be noted that the use of radioactive elements is generally frowned upon
by today’s society because of the difficulty in safely disposing of them. The main
advantage of this type of detector is its simplicity of manufacture and low cost.
General Optical Detectors
The standard used for measuring the concentration of smoke in air is the amount of
attenuation it will cause in the intensity of a light beam shining through it, usually
expressed as ‘percent obscuration per metre’. When the smoke is invisible, such as that
which can effect an ionisation detector, then a correlation is derived between the
different types of smoke, visible and invisible. The measurement units for optical
detection systems always end up as ‘percent attenuation per unit length’ because they
can be translated into an accurate laboratory measurements.
The different principles of Optical detection are described below.
A simplified adaptation of the laboratory meter described above immediately becomes
interesting as a smoke detector. The laboratory instrument itself is costly to manufac-
ture, and is impractical as a commercial smoke detector due to calibration problems
in an industrial situation. In order to make it commercially viable, the specification
must be changed so that dust build-up etc. does not cause unwanted alarms. A large
attenuation of the beam is required (typically 30 - 50%) and consequently it needs to
cover large areas. The attenuation in light is achieved by several means, but a
commonly used method is to shine a collimated light beam through many metres to
a reflector from which the light is reflected toward a light sensor near the beam source.
Smoke occurring in the beam will attenuate its intensity, and this reduction in intensity
will be used to trigger an alarm. Such a system is also good at detecting mist, steam
or dust, also anything that may physically interrupt or reduce the beam intensity. Dirt
build-up on lenses and reflectors have been known to cause serious problems, as have
normal movement in buildings caused by temperature changes and wind.
Beam Detectors
